Add /design-system endpoint

This is where the representation of the design system will reside.
When the application runs in production (with DEBUG = False) the url
will return 404.

src/templates/design_system.pug

@@ -0,0 +1,7 @@
+| {% extends 'layout.html' %}
+
+| {% block body %}
+.row.py-2
+    .col.text-center
+        h1 Design System goes here
+| {% endblock %}